Generally, a metallic antenna interferes with a radio wave existing in the surroundings even when the metallic antenna is not used, thereby causing, scattering of the radio wave. For this reason, in a case in which a plurality of antennas are disposed in a region having a shorter length than the wavelength of a radio wave to be used, the antenna characteristics get worse due to the interference between adjacent antennas. Therefore, it is desirable that the degree of interference between an antenna and a radio wave is small at a time when the antenna is not used.
To this end, conventionally, an antenna device in which an antenna is not formed of a metallic material, and is made to be transparent electrically at a time when the antenna is not used, thereby reducing the interference with a radio wave at a time when the antenna is not used, has been proposed (for example, refer to Patent Literature 1). In the conventional antenna device disclosed in Patent Literature 1, a dielectric tube having a pair of electrode terminals arranged in a place near an earth plate, and containing an ionizing gas between the electrode terminals is disposed, and electric power is supplied from an external power supply to the pair of electrode terminals. As a result, the ionizing gas which enters a plasma state acts as a conductor for high frequency radio waves. In this state, by feeding a high frequency current from the high frequency power supply to the pair of electrode terminals, the dielectric tube operates as an antenna. Due to this configuration, since the dielectric tube serves as a simple dielectric substance at a time when the antenna device is not used, even in a case in which another antenna is disposed adjacent to the antenna device in the surroundings, the antenna device does not become an interference source providing a significant degree of interference for the other antenna.